This invention relates to casting of metals, and more specifically to a casting furnace and a method for continuous casting of magnesium and magnesium alloys.
The following steps are involved in conventional processes for the preparation and casting of magnesium and magnesium alloys.
1. Pure Magnesium Refining
This step is used to purify magnesium which has been produced by an electrolytic process and typically contains a number of impurities. The terms xe2x80x9cpure magnesiumxe2x80x9d and xe2x80x9cprimary magnesiumxe2x80x9d are usually used to describe unalloyed, unrefined magnesium at it comes to the foundry from the electrolytic process. Refined magnesium is also referred to as xe2x80x9cpurified magnesiumxe2x80x9d by the industry. Refining of pure magnesium may be performed in a continuous refining furnace as disclosed in Canadian Patent No. 1,179,150 (Wallevik et al.) or Canadian Patent No. 1,022,978 (Kosarev et al.). Refining furnaces are divided into a number of compartments, each of which contains a layer of molten magnesium in contact with an underlying layer of molten salt. Impurities separate into the salt layer and settle to the bottom as sludge. The partition walls separating adjacent compartments are provided with passages through which the magnesium passes, such that the purity of the magnesium becomes progressively higher as it magnesium passes through the successive compartments. In such furnaces, mixing of the magnesium layer and the underlying salt layer is encouraged to maximize the purification of the raw magnesium. To this end, the furnace disclosed in the Wallevik et al. patent includes downwardly directed passages in the partition walls which force the magnesium exiting each compartment downwardly to mix with the salt layer in the next compartment. Where no alloying elements are to be added, the refined magnesium may be pumped directly from the refining furnace to the casting line.
2. Alloy Preparation
2.1 Primary Magnesium Alloys
This step is conducted in an alloy preparation furnace. The capacity of the furnace is limited by several factors, including structural limitations, heat distribution and melt homogenization. The most economical furnace metal holding capacity is about 4 to 8 tons. Alloying is necessarily a batch process in which primary magnesium is charged to the furnace, and then alloying elements are added and the mixture is agitated. During melt agitation, manganese chloride is added for iron removal and flux is added for oxide removal. Agitation is then discontinued and the impurities are allowed to settle out of the melt. Batch analysis is then conducted and after the alloy is acceptable, it is ready for casting, with no further refining being required.
2.2 Recycled Magnesium Alloys
Recycled magnesium alloys are usually produced from magnesium scrap. The recycling process typically involves scrap inspection, sizing, pre-heating, melting, refining and casting. The melting and refining operations may utilize either flux based or fluxless technologies. Some melting and refining operations utilize blending of the recycled alloy with primary magnesium alloy produced from electrolytic magnesium metal or from magnesium metal produced by thermal processes. The refining operation is utilized to remove oxides and other non-metallic inclusions from the recycled metal. The recycling process may also include adjustment of chemical composition of the alloy.
3. Alloy Casting
The alloy is transferred from the alloy preparation furnace to a crucible casting furnace where it is typically cast into ingots. As with the alloying furnace, the size of the casting furnace is limited for structural and other reasons, and typically provides little or no buffer capacity. Accordingly, alloy casting is essentially a batch process.
Attempts to continuously transfer the alloy from the alloy preparation furnace to the casting furnace have in the past been unsuccessful, partly for the reason that turbulence created by transferring the molten alloy has a detrimental effect on the quality of the ingots. Accordingly, casting of magnesium alloys is essentially a batch process which is characterized by low productivity and high energy consumption.
Continuous refining furnaces such as that described by the above-mentioned Wallevik et al. patent have also been employed as casting furnaces for pure or alloy magnesium. Although such furnaces provide buffer capacity and therefore allow continuous casting of magnesium, the use of such furnaces for casting is inefficient for a number of reasons. Firstly, such refining furnaces are typically of a large size, capable of storing 25 to 35 tons of magnesium, whereas a typical casting line typically consumes magnesium or magnesium alloy at a rate of about 2 to 5 tons/hr. Thus, the use of such furnaces for casting requires the accumulation and heating of excessive amounts of magnesium. Secondly, when such furnaces are used for casting alloy magnesium, the molten metal fed to the furnace is already refined, and therefore subjecting it to further refining is unnecessary and wasteful.
Therefore, the need exist for a continuous casting furnace which can more efficiently cast pure magnesium and magnesium alloys.
The present invention overcomes the above-mentioned problems of the prior art by providing a method and a casting furnace for continuous casting of molten magnesium and magnesium alloys (primary and recycled). The casting furnace according to the invention comprises at least two compartments, with pure magnesium or magnesium alloy being charged into a feed compartment, and being removed for casting from an extraction compartment located downstream of the feed compartment. In each compartment, a layer of molten magnesium is in contact with a liquid heating medium, preferably a molten salt layer. However, to minimize mixing between the magnesium and molten salt layers, the present invention provides means for directing the magnesium along the magnesium/salt interface so as to minimize further mixing of the magnesium with the molten salt.
Furthermore, the compartments of the furnace according to the invention are separated by at least one vertical weir, over which the molten magnesium flows from the feed compartment where it is introduced to the extraction compartment from which it is removed. The weir is essentially free of openings between its top and the magnesium/salt interface, thereby further minimizing mixing of the two layers. Also, the weir isolates the extraction compartment from turbulence caused by introduction of the magnesium into the feed compartment.